Mianningite, (square,Pb,Ce,Na) (U4+,Mn,U6+) Fe-2(3+)(Ti,Fe3+)(18)O-38, a new member of the crichtonite group from Maoniuping REE deposit, Mianning county, southwest Sichuan, China

摘要

Mianningite (IMA 2014-072), ideally (square, Pb, Ce, Na)(U4+, Mn, U6+) Fe-2(3+)(Ti, Fe3+)(18)O-38, is a new member of the crichtonite group from the Maoniuping REE deposit, Mianning county, Sichuan province, China. It was found in fractures of lamprophyre veins and in the contact between lamprophyre and a later quartz-alkali feldspar syenite dyke with REE mineralization, and is named after its type locality. Associated minerals are microcline, albite, quartz, iron-rich phlogopite, augite, muscovite, calcite, baryte, fluorite, epidote, pyrite, magnetite, hematite, galena, hydroxylapatite, titanite, ilmenite, rutile, garnet-group minerals, zircon, allanite-(Ce), monazite-(Ce), bastnasite-(Ce), parisite-(Ce), maoniupingite-(Ce), thorite, pyrochlore-group minerals and chlorite. Mianningite occurs as opaque subhedral to euhedral tabular crystals, up to 1-2mm in size, black in color and streak, and with a submetallic luster. Mianningite is brittle, with a conchoidal fracture. Its average micro-indentation hardness is 83.8 kg/mm(2) (load 0.2 kg), which is equivalent to similar to 6 on the Mohs hardness scale. Its measured and calculated densities are 4.62 (8) g/cm(3) and 4.77 g/cm(3), respectively. Under reflected light, mianningite is grayish white, with no internal reflections. It appears isotropic and exhibits neither bireflectance nor pleochroism. The empirical formula, calculated on the basis of 38 O atoms per formula unit (apfu), is [square(0.322)(Pb0.215Ba0.037Sr0.036Ca0.010)(Sigma 0.298)(Ce0.128La0.077Nd0.012)(Sigma 0.217) (Na0.127K0.036)(Sigma 0.163)](Sigma 1.000)(U0.4474+Mn0.293U0.1126+Y0.091 Zr0.023Th0.011)(Sigma 0.977)(Fe1.2243+Fe0.2432+Mg0.023P0.008Si0.006 square 0.496)(Sigma 2.000)(Ti12.464Fe5.2923+V0.1185+Nb0.083Al0.026Cr0.0173+)(Sigma 18.000)O-38. Mianningite is trigonal, belongs to the space group R (3) over bar, and has unit-cell parameters a = 10.3462(5) angstrom, c = 20.837(2) angstrom, V = 1931.65 (20) angstrom(3), and Z = 3. The structure was solved (